Optical coupling elements of the above-mentioned type are employed to optically couple an optical sensor located in the interior of a vehicle to the window pane of the vehicle. In order to ensure the reliable functioning of the sensor, it is desirable for the refractive index of the material used for the coupling element to be the same as the refractive index of the window pane of the vehicle. Furthermore, it is preferred for the material used for the coupling element to have the highest possible transmittance or permeability to light of the relevant wavelength. In this context, this can be light in the visible or invisible spectral range, depending on the application case.
In view of the above-mentioned requirements, the state of the art makes use of silicone materials as the materials for the coupling element. Less highly cross-linked silicone materials, so-called silicone gels, or else more highly cross-linked silicones, hereinafter referred to as silicone elastomers, can be employed.
An example of a silicone gel is disclosed in European patent specification EP 2 181 023 B1. SILGEL 612 is a trade name for such a silicone gel. Such materials are commercially available, for instance, from the BAYER company or from the WACKER company. These less highly cross-linked silicone gels are gelatinous, in other words, not inherently stable. For this reason, such coupling elements are produced “in situ”, that is to say, as a free-flowing and then curing material that is placed into a space in the housing of the optical sensor. The sensor housing is subsequently attached to the window pane, whereby the silicone gel comes to rest against the window pane. Such a production technique is demanding, both at the time of the initial installation and when a vehicle window pane has to be repaired. When a vehicle window pane or a sensor is repaired, it is also the drawback that the coupling element, which is not inherently stable, is destroyed and therefore cannot be re-used. Silicone gels, however, entail the advantage that their good optical properties remain constant even after a prolonged service life, also in those cases when the vehicle is exposed to severe stresses, for example, direct sunlight over a prolonged period of time. Moreover, they are intrinsically very soft and therefore can easily be adapted to the contour of the window pane.
Somewhat less soft silicone gels are known, for instance, under the trade name SEMICOSIL 912, made by the WACKER company. These silicone gels are likewise produced “in situ”, as described above and, for example, in European patent application EP 1 027 204 A2. They entail the drawbacks already elaborated upon above.
Diverging from an “in-situ processing”, more highly cross-linked silicone elastomers can be prepared as molded parts that can be handled manually and that can thus be produced away from the place of installation and subsequently handled manually. Examples of such coupling elements are those manufactured by the PRECISION REPLACEMENT PARTS CORPORATION of San Diego, Calif., United States.
Since silicone elastomers that have not been plasticized are too hard for direct use, an oil can be added as a plasticizing component to the silicone formulation during the production of the coupling elements. This markedly improves the installation and application properties since the surfaces of the molded parts that rest against the optical sensor and against the inside of the vehicle window pane can adapt to the corresponding contact surfaces of the optical sensor and of the inside of the vehicle window pane. Such improved coupling elements are known by the registered trade designation SENSORTACKREADY and are made by the applicant.